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Internal Mechanism and External Factors of Life Attenuation of Power Lithium Battery

Analysis of internal mechanism affecting the life of lithium battery

Lithium battery converts chemical energy into electrical energy through normal chemical reaction. In theory, the reaction inside the battery is the redox reaction between the positive and negative electrodes. According to this reaction, the ion is de embedded to generate current, so the lithium ion concentration will not change. However, in the actual battery cycle, in addition to the normal reaction, lithium ion will also have many side reactions, such as the formation and growth of SEI film, the decomposition of electrolyte, etc. Any reaction that can produce lithium ions or consume lithium ions will destroy the internal balance of the battery. Once the balance changes, it will have a serious impact on the battery. The internal factors of the battery that cause the decline of lithium battery capacity and service life are as follows:

1. Change of positive material

The dissolution and structural change of cathode materials are important reasons for the change of cathode materials. With the increase of battery cycle times, lithium ions are constantly de embedded in the positive electrode, which leads to the expansion and contraction of the lattice volume of the positive electrode material, changes the structure of the positive electrode material, and reduces the ability of the positive electrode material to de embed lithium ions. However, the dissolution of positive electrode materials generally occurs in the case of deep discharge. The dissolution of the positive material will generate a solid film attached to the surface of the positive material, preventing the insertion and removal of lithium ions, which will also make the battery capacity decline.

2. Electrolyte decomposition

The positive electrode will generally be decomposed into insoluble products such as lithium fluoride, which will block the pores, consume active substances and weaken the battery capacity. The decomposition voltage of the positive pole is usually greater than 4.5V, so the electrolyte is not easy to decompose at the positive pole, the electrolyte is not stable in the graphite layer, which is easy to cause side reactions, reduce the lithium ion concentration in the battery, and weaken the battery capacity. The electrolyte is easy to undergo reduction reaction at the negative pole when charging, which will cause the decomposition of the electrolyte and the reduction of the solvent. During the initial charge and discharge, a passive film (SEI film) will be formed inside the battery, Prevent further oxidation of electrolyte and cathode, which will adversely affect battery capacity.

3. Formation and growth of SEI film

The SEI film is formed during the initial charging and discharging process of the battery, which will consume some active substances. The SEI film can protect the negative pole of the battery, prevent the direct contact reaction between the electrolyte and the negative pole of the battery, prevent the loss of active lithium, and thus increase the cycle life of the battery. However, in the subsequent cycle, as the electrode material continues to expand and contract, new locating points are exposed, and the SEI film will continue to grow with the exposure of new locating points, which will cause the continuous loss of lithium ions, and the macroscopic performance is the capacity decline.

4. Formation of lithium dendrite

If the battery is charged at a current higher than its rated current for a long time or charged and discharged at a low temperature, its cathode is easy to form lithium dendrites. This metal lithium dendrite is easy to pierce the diaphragm, which will make the positive and negative electrodes of the battery directly contact, thus causing internal short circuit of the battery. This is a destructive failure of the battery, and the lithium dendrite is difficult to detect before the battery is short circuited.

5. Effect of inactive ingredients

As the number of the battery cycles increases, the adhesive will decompose inside the battery, which makes the active substances inside the battery constantly fall off, reducing the active substances involved in the positive and negative electrode reactions. The collecting fluid will also corrode after several charging and discharging cycles. The corrosion products will inactivate the active materials, increasing the internal resistance of the battery. The internal failure mechanism of lithium battery is mostly caused by the formation of lithium dendrite, the change of cathode material and the decomposition of electrolyte. In particular, the formation of lithium dendrite is easy to cause short circuit, which will lead to the thermal runaway of the electric core. Poor control will lead to the explosion of the electric core.

In the final analysis, the failure research of lithium battery is to study the failure mode and mechanism of the battery, optimize the battery and improve the battery safety. Therefore, the research on battery failure can not only guide the actual production and operation, but also improve the battery life, the safety and reliability of electric vehicles, and reduce the use cost of electric vehicles.

Analysis of External Factors Affecting the Life of Lithium Battery

The research shows that the external factors affecting the capacity and life decay of lithium batteries include temperature, charge discharge ratio, etc., which are all determined by the user's operating conditions and actual working conditions. The following external factors affecting battery aging are the most common.

1. Discharge depth DOD

Discharge depth refers to the percentage of the battery discharged from the full state to the rated capacity of the battery. When the battery is discharged to the cut-off voltage, the battery discharge rate is 100% DOD. 60% DOD means that the battery is between 100% SOC and 40% SOC. The greater the depth of battery discharge, the greater the amount of electricity discharged. The research shows that the increase of AC internal resistance during charging and discharging is relatively small when the battery is used in DOD (20%~80%), while deep discharge will increase the internal resistance of the battery, thus reducing the service life of the battery.

2. Overcharge

When the battery is overcharged, the lithium ion on the cathode has already reached saturation, but if the lithium ion continues to be embedded, the lithium ion will be deposited on the surface of the cathode, blocking the porous material of the cathode, making it more difficult for the lithium ion to be de embedded, and reducing the concentration of lithium ion in the battery. The macroscopic performance is the loss of battery capacity. Overcharging will lead to the increase of battery voltage. When it is higher than a critical value, the electrolyte will oxidize to generate insoluble substances and gases, which will block the pores of porous materials and reduce the ion transmission rate. Generally, the charging cut-off voltage and charging cut-off current are set to prevent the battery from overcharging. Whether nickel metal hydride battery or lithium battery, when overcharging occurs, a large amount of heat energy from current conversion will be emitted, leading to the occurrence of many reactions inside the battery, such as the reaction between the positive and negative electrodes and the electrolyte, which reduces the maximum capacitance of lithium battery. When the heat is not easily dissipated when it accumulates, it may even cause fire and explosion.

3. Self discharge

Lithium ion power lithium battery will self discharge. Usually, the self discharge is shown by the loss of battery capacity. Most of the self discharge is reversible, but there is still irreversible self discharge. There are many reasons for irreversible self discharge, such as the loss caused by the irreversible reaction of lithium ions, the increase of internal resistance caused by the blockage of pores by insoluble substances generated by the oxidation reaction of electrolyte, and the rise of SEI film. Such chemical reactions will reduce the lithium ion concentration inside the battery, thus causing capacity degradation.

4. Ambient temperature

The performance of lithium battery will change when the temperature is too high or too low. Too low temperature will affect the activity of electrolyte inside the battery, reducing the charging and discharging efficiency of the battery. Too high temperature will damage the chemical equilibrium system inside the battery. Many irreversible side reactions will occur in the battery under high temperature, which will deform the electrode structure of the battery, reduce the battery capacity, and reduce the number of battery cycles.

5. Pressure

Pressure. In order to facilitate the diffusion of lithium ions in the battery, the separator and positive and negative electrodes of lithium batteries usually have a porous structure, and the pressure will have a certain impact on the porosity and tortuosity of porous materials. Therefore, the mechanical pressure will indirectly affect the diffusion rate of lithium ions between the positive and negative electrodes and the separator, thus affecting the discharge performance of lithium batteries. If the pressure is not applied, the battery will be difficult to fix, but too much pressure will make the gap between the negative graphite layers too small, resulting in an increase in the van der Waals force between layers, an increase in the insertion resistance of lithium ions, and a corresponding decrease in the number of lithium ions embedded, which will cause a decrease in capacity. Therefore, it is necessary to study the battery pressure.

reference:

Zeng Yunlu. Hunan University. Study on the influence of pressure on the performance and life of soft packed lithium battery

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